1. Field of the Invention
The present invention relates to a controller for a vehicle automatic transmission, which engages or disengages, in a rotational state, plural hydraulic engagement elements operated by a hydraulic pressure to switch plural gear positions, defined based on the traveling conditions of the vehicle, automatically according to predetermined gear shifting characteristics.
2. Description of the Related Art
This kind of controller controls the feed and discharge of working oil to and from hydraulic clutches as plural hydraulic engagement elements provided in a transmission to store, in an electronic control circuit, gear shifting characteristics obtained by setting, for example, throttle openings of an engine (or accelerator pedal openings) and vehicle speeds as parameters. Then, the controller engages or disengages the hydraulic clutches to perform automatic gear shifting in order to effect gear positions according to the gear shifting characteristics.
This kind of hydraulic clutch includes a hydraulic chamber to which working oil is supplied, and a piston for bring plural friction plates into pressure contact with each other by a hydraulic pressure of the hydraulic chamber. The hydraulic chamber and the piston are provided on a rotating shaft to rotate integrally with the rotating shaft. The feed and discharge of working oil to and from the hydraulic chamber is done through an oil passage provided on the rotating shaft. When the working oil is supplied to the hydraulic chamber, the piston presses the plural friction plates to frictionally engage the friction plates. On the other hand, when the working oil is discharged from the hydraulic chamber, the pressing force of the piston is released to make the plural friction plates enter a disengaged state. However, upon shifting from the engaged state to the disengaged state while they are rotating, the working oil may not be discharged smoothly from the hydraulic chamber due to its centrifugal force. In this case, the pressing force of the piston is not released due to centrifugal hydraulic pressure in the hydraulic chamber. As a result, the friction plates become a half-engaged state (so-called dragged state), and this may reduce the durability of the hydraulic clutches due to heat developed in the friction plates, and the like.
To solve this problem, there is known a conventional structure, which provides, in each hydraulic clutch, a centrifugal pressure canceller chamber opposite to the hydraulic chamber to supply working oil to the centrifugal pressure canceller chamber at the time of discharging the working oil from the hydraulic chamber in order to cancel the centrifugal hydraulic pressure generated in the hydraulic chamber (for example, see Japanese Patent Application Publication No. 8-28591).
However, if the centrifugal pressure canceller chamber is provided for all the hydraulic clutches included in the transmission, not only the structure of the transmission becomes complicated, but also the transmission cannot be made compact. Therefore, in general, the centrifugal pressure canceller chamber is provided only for a low gear range of hydraulic clutches, not provided for a high gear range of hydraulic clutches, to prevent the transmission from becoming a large size. Further, if normal gear shifting characteristics are used in this structure, since a shift to a low gear position is performed while keeping a high-speed state of the vehicle upon shifting a middle gear position to the low gear position, hydraulic clutches for a high gear range rotates at high speed to cause a dragged state. To avoid this, the hydraulic pressure of the hydraulic clutches needs to be so released that the hydraulic clutches for the high gear range do not rotate at high speed. In other words, a gear shifting characteristic from the middle gear position to the low gear position is set to a low vehicle speed side to shift to the low gear position after the vehicle speed is sufficiently slowed down at the middle gear position. Then, after shifting to the low gear position, the rotation of the hydraulic clutches for the high gear range is suppressed to release the hydraulic pressure of the hydraulic clutches.
However, such a setting of gear shifting characteristics as to give priority to protection of the hydraulic clutches result in holding the middle gear position for a relatively long time until the vehicle speed is sufficiently slowed down after shifting from a high gear position to the middle gear position during climbing a slope. In this case, sufficient acceleration cannot be obtained due to a delay of downshift. Further, even during sports run on a winding road without entering the high gear range at all, since the gear shifting characteristics are set to the low vehicle speed side, sufficient acceleration cannot be obtained due to a delay of downshift, causing a problem of making driveability worse.